Post/Author/DateTime	Post
#1 dragonsblood Feb 12, 2009 17:31:42	I'm creating a homebrew world and I want it to have two moons. I'd like its physics to be as real as possible. I know the moon in real life is what creates the tides, so what would happen if we had a second, closer moon?
#2 Novacat Feb 12, 2009 18:18:31	I'm creating a homebrew world and I want it to have two moons. I'd like its physics to be as real as possible. I know the moon in real life is what creates the tides, so what would happen if we had a second, closer moon? It would fall into the planet. That is, if we assume that one moon is the same size and distance as Luna is from Earth. If we don't assume that, then I would suppose that the closer moon would be the most determining factor in the effects on tidal patterns, so there would be relatively little change from Earth's tidal patterns. Another possibility is that both moons are equidistant, orbiting in opposite directions, each a few degrees north or south of the planet's equator. Their orbiting patterns could be truly unique in this situation, fully changing direction twice on each orbit due to their interaction with each other: What effect this would have on the tides of the planet I can't even guess, but it sure would be interesting.
#3 rikiji Feb 12, 2009 18:27:38	Some food for thought http://scubageek.com/articles/wwwtides.html http://curious.astro.cornell.edu/question.php?number=247 http://www.synapses.co.uk/astro/moon1.html On a practical level, unless you're already studying the applicable realms of astronomy and physics it's too much work for a system that your players will probably never care about. Tides are actually caused by the moon and (too a lesser degree) the sun - other planetary bodies also have minor effects, but generally too small to measure. Hypothetically, the tides would be both more intense and more erratic - the differing lunar cycles would synergize and some points and cancel at others. The time period for a complete tidal cycle will be roughly equal to the least common denominator of the orbital periods - which could be huge (with an additional contributory factor by the sun for fluctuations within the cycle). That said, how likely is it that your players are going to notice the tidal cycle unless you cram it down their throats? Right.
#4 doomsought Feb 12, 2009 19:05:04	You have 3 circular sheets of paper connected in the middle. The first sheet represents the planet around the moons' axis of rotation. the two other sheets have numbers, you add them to get the tide. Rotate the sheets at different but constant rates. If you mark the moon you could probably tell them its phase as well.
#5 vebrandsson Feb 12, 2009 19:13:55	Well if you want to do the math, the moon causes the tides to rise and fall in a perfect sine wave pattern. To solve the issue with two moons you would calculate the sine wave pattern for each moon individually as if the other did not exist and then you would add the two functions together to get the end result in the tides. Depending on if the moons orbit at the same rate or how they align the results can vary greatly.
#6 holy_beholder Feb 14, 2009 15:12:46	That is to say, if they orbit at the same rate, the tides will be much more cyclical, where as at different rates there will be times with higher peaks, others with lower, etc. If you've seen a biorhythm calculator that does totals you would see what I'm talking about.
#7 mellored Feb 15, 2009 8:46:18	Sounds fun, you can easily have a "cave is only accessible once every hundred years" things (ignoring underwater breathing). Or a "moons align and a whole country flood".
#8 vebrandsson Feb 15, 2009 13:19:35	That is to say, if they orbit at the same rate, the tides will be much more cyclical, where as at different rates there will be times with higher peaks, others with lower, etc. If you've seen a biorhythm calculator that does totals you would see what I'm talking about. Actually thats a brilliant idea, something like a biorhythim calculator would be a good example of the effects.
#9 awestriker Feb 15, 2009 15:50:25	First off: it's a fantasy world. Gravity can work slightly differently and you can even ignore the Roche limit and other physical stuff. Perhaps one of your moons is geosynchronous and a great temple to Sehanine (or whoever your moon deity is) is built in the location immediately below it. Whatever the case, be sure to give them distinguishing features and different names. (unless the feature is that they are exactly identical. That counts.).
#10 Daybreaker Feb 15, 2009 16:21:18	Some planets do have multiple moons, but those planets seem to be very big and their moons are (by comparison) very small. It might not work out very well for a more Earth-sized planet to have multiple moons, unless they were also very small, like the moons of Mars. They're so small they're not even remotely spherical. But there's no reason to say that it couldn't happen. It might be hideously unlikely for an Earth-sized planet to have two Luna-sized moons, but I think astronomers consider it weird that Earth has even one moon the size of Luna. Luna's size as compared to Earth's is unusually big, for a moon. So, obviously, weird variations do happen. My mostly uneducated guess is that for two Luna-sized moons to be orbiting an Earth-sized planet, one would have to be somewhat bigger than the other, and the smaller one would have to be in a closer orbit. I'll even go so far as to suggest that you balance out the appearance of size, so that the smaller, closer moon appears nearly identical in size to the further, larger moon. This gives you the opportunity for dramatic eclipses, and it also seems to be the way things like to work out on Earth-sized planets (note that the size of the moon and the size of the sun are very similar from the perspective of Earth -- maybe not a mathematical coincidence? I don't know). I think that the effect on the tides would be dramatic, but endurable. In the case of the tides, you're going to have the larger moon causing most of the tides, and the smaller moon will be either dampening or exaggerating those tides. So sometimes a low tide will be very low, and sometimes a high tide will be very high. Beaches in general would be wider. Ocean currents might be stronger. This would be a world where the ocean has more energy. As for the effects on women ...? Probably similar to the tides. Beyond that, I dare not speculate.
#11 vebrandsson Feb 15, 2009 19:09:47	Do note that there are other things to consider besides tides with a two moon planet. As the poster above me mentioned it would strongly effect sea currents, which would greatly effect sea trade as well as sea life. Also what effect on creatures like lycanthropes would be interesting as well as anything else that happens on a lunar cycle, the 12 month callendar we use is a roughly lunar based callendar for instance, but when you have two moons that doesn't work as logically.
#12 Neutronium_Dragon Feb 16, 2009 6:39:07	> It might not work out very well for a more Earth-sized planet to have > multiple moons, unless they were also very small, like the moons of Mars. > They're so small they're not even remotely spherical. And falling out of orbit because of the problems mentioned above. In a fantasy world though, the effect on the tides is really whatever you want it to be. Eberron has twelve(thirteen) moons and a ring, and its tides aren't called out as being unusual in any way.
#13 holy_beholder Feb 17, 2009 21:56:51	Actually thats a brilliant idea, something like a biorhythim calculator would be a good example of the effects. Thank you. That said those things are a pain to find, I was able to find a few with a graph, but none with a total, where the total would show the effects really well. But I didn't give it a huge amount of effort, and I've seen ones that total stuff up before.
#14 etarnon Feb 17, 2009 22:28:43	The dirt simple easiest way to do this is to figure out how long a lunar month is, and then as others have said, it's a sine wave between say day 0 Max, day 14 min, day 28/0 max. Then figure out how long your other moon cycle is, and then as said before add. So if your moons are in a harmonic cycle, such that one moon is 28 days, and the other is 56 days... every 56 days you'd have a super max high tide. then lesser tides as you go. More complicated: break out Traveller's SCOUTS book 6, and calculate it all out.